Three siblings and their cousin have arrived at their grandparents' farm to spend their summer vacation. While exploring far into the woods, they find what looks like an old door seemingly built into the side of a large mound of earth. On opening the door, they descend into a mysterious tunnel where they find hidden gold, their friend and neighbor, and his diving partner held captive by two thugs who are determined to find the gold. In the end, the children rescue their friends and are rewarded for their bravery.